The advent of "smart" circuit interrupters, such as described within U.S. Pat. No. 4,672,501 permits several such circuit interrupters arranged within a common enclosure to communicate with a central processor unit over a power line communication network or a twisted pair of conductors. The central processor is in the form of a field programming unit which continuously interrogates each of the interconnected circuit interrupters to determine the status of the individual circuit interrupter setpoint parameters as well as to determine whether a circuit interrupter has responded to interrupt circuit current to an associated piece of electrical equipment. An operator could then readily determine from the field programming unit which of the circuit interrupters has responded and the nature and magnitude of the overcurrent condition causing the circuit interrupter to respond.
In certain industrial environments, it is customary to assemble a main circuit interrupter along with several branch circuit interrupters within a common enclosure such as within low voltage switchgear applications. The field programming unit is arranged within the enclosure and is interconnected with the main and branch circuit interrupters by means of a communications link to allow the condition and status of each of the circuit interrupters and the associated protected electrical equipment to be continuously monitored. The branch circuit interrupters communicate with each other, the field programming unit and the main circuit interrupter for a variety of functions including Zone Selective Interlock such as described, for example, in U.S. Pat. No. 4,468,714. With Zone Selective Interlock function an upstream circuit interrupter is delayed from operating to interrupt circuit current to allow a downstream circuit interrupter closer to the fault to interrupt the circuit. The trip-time characteristics of the individual branch circuit interrupters can be independently adjusted to insure that the circuit interrupter closest to the fault will interrupt before any of the upstream circuit interrupters. The use of such a communications link between the main and branch circuit interrupters within a common enclosure was heretofore limited to so-called "medium voltage" circuit interrupters having a voltage rating in the order of ten or more killovolts. The proposed use of electronics in "low voltage" circuit interrupters in the order of several hundred volts, would allow such low voltage circuit interrupters to communicate with each other by means of a similar communications link.
One purpose of the invention accordingly is to provide means for communication between low voltage circuit interrupters without affecting the circuit interrupter overcurrent determination.